For example, a by-wire type brake system that generates a braking force via an electrical circuit is employed in a hybrid vehicle, in addition to a conventional brake system that generates the braking force via a hydraulic circuit. Such a by-wire type brake system converts an operation amount of a brake pedal by a driver to an electrical signal, and transmits the signal to an electric motor that drives a piston of a slave cylinder. Then, a brake hydraulic pressure boosted by a driving of the piston by the electric motor is generated in the slave cylinder, and the generated brake hydraulic pressure operates wheel cylinders to generate the braking force (for example, see Patent Document 1).
In the by-wire type brake system according to Patent Document 1, if the slave cylinder fails while the wheel cylinders are operated by the brake hydraulic pressure generated by the slave cylinder during normal operation, the brake hydraulic pressure in the wheel cylinders is maintained by closing for a predetermined time an opening and closing valve disposed on a fluid passage connecting the slave cylinder to the wheel cylinders.
According to a fail-safe technology according to the Patent Document 1, it is possible to sufficiently raise the brake hydraulic pressure which is generated by a master cylinder for backup during the predetermined time. Therefore, it is possible to prevent the brake hydraulic pressure in the wheel cylinders from decreasing suddenly, when a braking by the slave cylinder is switched to the braking by the master cylinder.